The primary sites of hematopoietic cell differentiation during embryogenesis in the amphibian, Rana pipiens, appears to be homologous to the sites of hematopoiesis in the higher vertebrates. Differentiation of functional T-cells occur within the thymus; preliminary morphological evidence suggests that functional B-cells may differentiate within the liver; and granulocyte differentiation probably occurs within the pronephros. Evidence obtained from the reciprocal transplantation of primitive tissue anlagen between 72-hour embryos of chromosomally labeled leopard frogs suggests that the putative hematopoietic stem cell differentiates extremely early in this species and migrates into the appropriate regions of the embryo prior to overt histological differentiation of the particular organs. Thus, one question under investigation involves the early embryonic derivation of the hematopoietic stem cell and its subsequent migratory pathways. The inter-relationships between various sites of hematopoiesis are also being investigated. For example, preliminary studies using chromosomally labeled anlagen transplants indicate that in the absence of the thymus, the embryonic ventral blood islands make a major contribution to the peripheral lymphoid organs. However, in the presence of the thymus the lymphoid contribution from this region is negligible.